Method and system for bonding 3D semiconductor devices

ABSTRACT

A method and system and for fabricating 3D (three-dimensional) SIC (stacked integrated chip) semiconductor devices. The system includes a vacuum chamber, a vacuum-environment treatment chamber, and a bonding chamber, though in some embodiments the same physical enclosure may serve more than one of these functions. A vacuum-environment treatment source in communication with the vacuum-environment treatment chamber provides a selected one or more of a hydrogen (H 2 )-based thermal anneal, an H 2 -based plasma treatment, or an ammonia (NH 3 )-based plasma treatment. In another embodiment, a method includes placing a semiconductor chip in a vacuum environment, performing a selected vacuum-environment treatment, and bonding the chip to a base wafer. A plurality of chips formed as dice on a semiconductor wafer may, of course, be simultaneously treated and bonded in this way as well, either before or after dicing.

TECHNICAL FIELD

The present invention relates generally to the fabrication ofsemiconductor devices, and related more particularly to a system andmethod for bonding together the separate chips in a 3D(three-dimensional) semiconductor device such as a 3D-SIC (stackedintegrated circuit) SIP (system in package) device.

BACKGROUND

Modern electronic appliances are frequently manufactured using one ormore packaged semiconductor devices, which are sometimes called chips,each enclosed in a familiar black plastic package or in some other typeof protective enclosure. Each chip performs a specific functionassociated with the operation of electronic appliance, and is mounted onone or more printed wired boards (PWBs) that provide for mechanicalsupport and for electrical connections between the various chips. ThePWBs and one or more input and output devices such as keyboards or LCDscreens are then secured in some type of housing. Examples of modernelectronic appliances include mobile telephones, personal digitalassistants, and personal gaming devices. Larger electronic appliancesinclude personal computers and television sets.

A chip is, generally speaking, a small piece of semiconductor materialonto which a large number of tiny electric components have beenfabricated and connected with one another to form integrated circuits.An example is shown in FIG. 1. FIG. 1 is a simplified perspective viewillustrating a typical semiconductor chip 10. As can be seen in FIG. 1,semiconductor chip 10 is thin and usually in the shape of a square orrectangle. On the top surface 11 of semiconductor chip 10 is an activearea 12. Active area 12 is simply the portion of top surface 11 at whichthe many tiny electrical components (not shown) are located. In thisparticular example, active area 12 also includes a series of bond pads13, which are basically larger, externally-accessible conductive pads towhich exterior electrical connections may be made, for example byattaching a bond wire (see FIG. 3). A seal ring 14 is formed on surface11 of semiconductor chip 10 about the periphery of active area 12,segregating it from the rest of the chip and providing protection duringcertain fabrication operations. The backside 15 of chip 10 is referencedbut not visible in FIG. 1. Ordinarily there are no circuits or padsformed on the backside 15, although there may be in some applications.

For efficiency, the electronic components for a number of chips areoften fabricated simultaneously. For this purpose a thin wafer may besliced from an ingot of, for example, silicon, and treated by ionimplantation to impart semiconductor properties. An example is shown inFIG. 2. FIG. 2 is a plan view illustrating a typical semiconductor wafer20. For the purpose of illustration, chip 10 is pointed out, although atthis stage it has not yet been separated from the other, adjacentdevices, and is typically referred to as a die. As can be seen in FIG.2, a plurality of dice 22 is often formed in an array on the surface 21of wafer 20. The dice 22 are collectively fabricated in a series ofprocess steps that, in general, selectively deposit and selectivelyremove layers of insulating, conductive, and semiconductor material.Much of this process is automated, and great precision is required. Anorientation notch 23 is for this reason formed in the periphery of wafer20 so that it's proper positioning may be confirmed. Other methods oforientation control are used as well. Inspection and various types oftesting take place at certain points in the fabrication process toidentify those dice containing irremediable defects so that they are notused. Each of the dice 22 are usually though not necessarily identicalwith respect to each other, and will later in the fabrication process beseparated into individual chips, such as the chip 10 shown in FIG. 1.The process for separating the chips is sometimes referred to as dicing.

As mentioned above, the various chips mounted on a given PWB (not shown)are often interconnected, and may in that manner form systems intendedto perform an overall function, or set of functions, with each chipperforming its own function-associated tasks. As small, energy-efficientelectronic appliances become more popular, however, there arises ademand for new ways to configure the systems on which they rely. Thechips forming an entire system, for example, may be contained within asingle package. An exemplary SIP (system in package) device is shown inFIG. 3.

FIG. 3 is a simplified perspective view illustrating an exemplary 3D(three-dimensional)-SIC (stacked integrated circuit) semiconductordevice 30. As should be apparent, this device consists of three chipsthat have been mounted one on top of the other. Semiconductor chip 10shown in FIG. 1 is in this example at the top of the three-chip stack.Semiconductor chip 31 is directly beneath it, and semiconductor chip 32is beneath a semiconductor chip 31. An intermediate layer 33 is shownbetween chip 31 and chip 32, and intermediate layer 34 is shown betweenchip 31 and chip 10. These intermediate layers are typically some typeof insulating material that may also serve to bond the chips together.When assembled, the entire device 30 may be encased in this or a similarmaterial, though this is not shown in FIG. 3. Semiconductor chip 10 isthe smallest of the three chips in device 30, and semiconductor chip 31the largest, permitting the numerous electrical connections requiredbetween the chips to be made using bond wires 35. As should be apparent,this is a very simplified illustration; an actual device may have dozensof such bond wires. Alternative means of providing electricalconnections may also be used, for example creating vertical conductivestructures that connect components on one chip with components onanother, usually adjacent chip.

Unfortunately, when using vertical conductive structures to makeconnections between the components of one semiconductor device and thoseof another, problems in bonding may occur. The processes used to prepareone chip for bonding with another, such as grinding and polishing, oftenleave behind chemical residue and deleterious material that interfereswith the bonding process. In addition, if too much time is allowed toelapse between preparation and bonding, oxidation or corrosion maydegrade the quality of the exposed conductors. Eliminating this delay(sometimes referred to as Queue-time, or simply Q-time), however, mayimpose addition costs or other undesirable consequences. Needed, then,is a manner of manufacturing semiconductor devices such as 3D-SICs, andin particular preparing the multiple chips involved for bonding, whichprovides for greater bonding reliability and permits longer Q-times. Thepresent invention provides just such a solution.

SUMMARY OF THE INVENTION

These and other problems are generally solved or circumvented, andtechnical advantages are generally achieved, by preferred embodiments ofthe present invention directed to fabrication of a 3D(three-dimensional) SIC (stacked integrated circuit) semiconductordevice. In accordance with a preferred embodiment of the presentinvention, a method for fabricating a semiconductor device includesforming a plurality of dice on a semiconductor wafer, placing the waferin a vacuum chamber and drawing a vacuum, performing a selectedvacuum-environment treatment, and then bonding the semiconductor waferto a base semiconductor wafer, preferably while maintaining the vacuumenvironment. In accordance with the present invention, the selectedvacuum-environment treatment includes one or more of a hydrogen(H₂)-based thermal anneal, an H₂-based plasma treatment, or an ammonia(NH₃)-based plasma treatment. In some embodiments, a chemical dip may beperformed prior to the vacuum-environment treatment, preferably using acitric acid or, alternately, a hydrochloric acid (HCl) bath.

In another aspect, the present invention is a method of preparingsemiconductor chip for bonding, comprising performing avacuum-environment treatment of the chip while it is in a vacuum orreduced-pressure environment. These may be performed in two chambers,the first for loading the wafer and drawing a vacuum, the second forperforming the vacuum-environment treatment itself. The chip in thisinstance is preferably transferred from the first chamber to the secondchamber without breaking vacuum. A third chamber may then be used forbonding the chip to a base wafer, or simply to another chip. In onepreferred embodiment, a chemical dip is performed prior to thevacuum-environment treatment.

An advantage of preferred embodiments of the present invention is thatoxidation of the exposed conductive material is reduced and undesirablemoisture and chemical residue is largely or completely expelled. Thisgenerally provides for better adhesion and, as a result, better bondingstability.

A further advantage of a preferred embodiment of the present inventionis that process Q-time is prolonged, easing time constraints otherwisepresent in the fabrication process without incurring significantadditional cost.

A more complete appreciation of the present invention and the scopethereof can be obtained from the accompanying drawings that are brieflysummarized below, the following detailed description of thepresently-preferred embodiments of the present invention, and theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 is a simplified perspective view illustrating a typicalsemiconductor chip.

FIG. 2 is a plan view illustrating a typical semiconductor wafer.

FIG. 3 is a simplified perspective view illustrating an exemplary 3D(three-dimensional)-SIC (stacked integrated circuit) semiconductordevice.

FIG. 4 is a flow diagram illustrating a method for fabricating asemiconductor device according to an embodiment of the presentinvention.

FIG. 5 is a simplified schematic diagram illustrating a system accordingto an embodiment of the present invention.

FIG. 6 is a flow diagram illustrating a method for fabricating asemiconductor device according to another embodiment of the presentinvention.

FIGS. 7 a through 7 h are a sequence of side views illustrating incross-section the configuration of a semiconductor device at variousstages of fabrication according to an embodiment of the presentinvention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the presently preferred embodiments arediscussed in detail below. It should be appreciated, however, that thepresent invention provides many applicable inventive concepts that canbe embodied in a wide variety of specific contexts. The specificembodiments discussed are merely illustrative of specific ways to makeand use the invention, and do not limit the scope of the invention.

The present invention will be described with respect to preferredembodiments in a specific context, namely a 3D (three-dimensional) SIC(stacked integrated circuit) semiconductor device. The invention mayalso be applied, however, to other semiconductor devices as well. Asmentioned above, the present invention is directed to a method andsystem for providing longer Q-times at certain stages of the chipmanufacturing process, and for reducing the risk of bonding failure byreducing the amount of moisture and residual chemicals remaining on thebonding surface after preparation for bonding. A system and methodsaccording to the present invention will now be described.

FIG. 4 is a flow diagram illustrating a method 100 for fabricating asemiconductor device according to an embodiment of the presentinvention. At START, it is presumed that the materials and equipmentnecessary to performing the method are available and operational. Thisprocess then begins with providing a wafer (step 105). The wafer may beformed, for example, of silicon or silicon-germanium, although othermaterials may be used. A plurality of dice is then formed on at leastone surface of the wafer (step 110). A plurality of dice is also formedon a second wafer (step 115). The second semiconductor wafer is thenplaced in a vacuum chamber (step 120) and a vacuum is drawn (step 125).In a preferred embodiment, the vacuum drawn in the chamber correspondsto a pressure differential of about 1 mTorr compared with atmosphericpressure.

In accordance with this embodiment of the present invention, avacuum-environment treatment is then performed (step 130). Note that asused herein, the term “vacuum-environment treatment” denotes that aselected treatment is performed while the second treatment is in avacuum environment. The selected treatment includes one or more ofhydrogen (H₂)-based thermal anneal, an H₂-based plasma treatment, or anammonia (NH₃)-based plasma treatment. The performance of one or more ofsuch treatment does not preclude, of course, the performance of otheroperations while the second wafer is in the vacuum environment.

The second semiconductor wafer and the first semiconductor wafer arethen bonded together (step 135). The bonding may be performed, forexample, by aligning the two wafers appropriately, forcing them togetherwith some force, and heating the assembly until certain intendedcomponents, such as copper conductors, have been joined together.Variations on this bonding operation are used in some alternateembodiments. The bonding operation, however, is preferably performedwhile the wafer assembly is in a vacuum environment. This implies, ofcourse, that at some point the first semiconductor wafer be introducedto the vacuum environment where the second semiconductor wafer islocated. It is preferred, however, that the second semiconductor waferbe kept in a vacuum environment throughout the process.

In another embodiment, the present invention is a system for fabricationof a 3D-SIC semiconductor device. FIG. 5 is a simplified schematicdiagram illustrating a system 200 according to an embodiment of thepresent invention. In this embodiment, system 200 includes a vacuumchamber 205 for producing a reduced-pressure environment. Avacuum-environment treatment chamber 210 is preferably in communicationwith vacuum chamber 205 so that one or more semiconductor chips orwafers placed in the reduced-pressure environment remain under vacuumcontinuously throughout this portion of the fabrication process. Avacuum-environment treatment source 215 is in communication with, orenclosed within, the vacuum-environment treatment chamber 210, andprovides the materials necessary for the selected vacuum-environmenttreatment at the appropriate time and in the appropriate manner. A heatsource 220 is present in this embodiment to raise the environmentaltemperature as necessary, but is not needed for all applications.Finally, a bonding chamber 225 is present, and preferably incommunication with vacuum-environment treatment chamber 210 so that thewafer or chip being treated remains under vacuum throughout this portionof the process. Heat source 220 may also provide an elevated temperaturefor bonding as well, and in this embodiment is for this reason incommunication with bonding chamber 225 as well. Alternately, a separateheat source (not shown) may be used instead.

Note that in the embodiment of FIG. 5, three separate chambers areshown. In other embodiments (not shown) a particular physical chambermay be used for performing additional operations. By the same token,additional components or treatment chambers may in some alternateembodiments be added to those shown in FIG. 5 while remaining within thespirit of the present invention.

FIG. 6 is a flow diagram illustrating a method 300 for fabricating asemiconductor device according to another embodiment of the presentinvention. Again, at START, it is presumed that all of the materials andequipment necessary to performing the method are available andoperational. A first wafer is provided (step 305), and populated with aplurality of dice (step 310). Note that although recited first, however,provision and population of the first wafer does not have to be startedat any particular time, or finished before the wafer is actually needed.Note also that die formation, as discussed herein, is intended to refergenerally to the formation of integrated circuits and related componentson a wafer surface, or multiple wafer surfaces. Each die represents asemiconductor chip and contains the necessary circuitry for the chip'sintended function.

The term chip will generally be used to refer to a completed die thathas been separated from the others formed on the wafer, although this isnot a strict requirement. That is, some operations in the methods of thepresent invention may be performed either before or after dicing, thatis, before or after separating the dice on the wafer into individualchips. The choice of terms, however, is not meant to imply a requirementthat a given operation must be performed at a specific time or thatseveral operations must be performed in a particular order. Although thechips produced from a single wafer are usually identical orsubstantially identical with respect to each other, this is notrequired. Here, the plural “dice” includes one die; a wafer may containonly one chip, although this would not usually be the case.

A second wafer is also provided, (step 315) and populated with dice(step 320). The second wafer is then, in this embodiment, bonded to acarrier (step 325). This protects the surface upon which the integratedcircuits are formed and leaves exposed the opposite, or what will bereferred to as the backside of the wafer. The unprotected backsidesurface is then reduced (step 330). This backside reduction thins thewafer, and may expose conductors disposed beneath the original backsidesurface, such as conductor-filled vias that are typically formed onlypart way through the wafer. This backside reduction may include, forexample, grinding, CMP (chemical-mechanical polishing), and etching,although these steps are not separately shown. The surface may be etchedback such that the via conductors, if present, actually protrudeslightly from the surface.

In the embodiment of FIG. 6, the dice are then diced into individualchips (step 335), for example by sawing, breaking, or laser etching. Tohold the chips in place during this process, a tape may be used (stepnot separately shown). In this case, the wafer is mounted on a tape, andthen the dice are sawed or broken apart but remained attached to thetape (which is removed at a later time). The good dice are then selected(step 340) through a process of inspection and testing, which is notshown and which may in fact involve several previously-performedoperations. Those chips determined not to be good are then removed anddiscarded (also not shown). A chemical dip is then performed (step 345),and although this procedure is useful, it is optional and is not done inall embodiments. When performed, the chemical dip is to be done in acitric acid or, less preferably, in an HCl bath.

According to this embodiment of the present invention, the remainingchips of the second wafer are now placed in a vacuum chamber (step 350),and a vacuum environment is created (step 355). A vacuum-environmenttreatment is then performed (step 360). The vacuum-environmenttreatment, according to the present invention, includes one or more ofan H₂-based thermal anneal, an H₂-based plasma treatment, or anNH₃-based plasma treatment. After the vacuum-environment treatment thefirst wafer is introduced into the vacuum chamber (step 360), preferablywithout breaking the vacuum. The chips (dice) of the first wafer arethen bonded to the remaining (known good) chips of the second wafer(step 365).

FIGS. 7 a through 7 h are a sequence of side views illustrating in crosssection the configuration of a semiconductor device 400 at variousstages of fabrication according to an embodiment of the presentinvention. FIG. 7 a shows base wafer 401, which includes a substrate405. An active area layer 410 is formed on the top surface of substrate405. Note here that references to the top and bottom, or upper and lowerportion of a given device are based on the orientation of the device inthe figure being described. In actual use, or even during fabrication,the device may assume a number of different orientations. In thisdescription, the term active area is generally used to describe thoseportions of the wafer upon which integrated circuits and relatedcomponents have been formed, and includes both the individual electroniccomponents that have been formed on and in the wafer surface and thelayers of dielectric-embedded metal that interconnects these components.A first-wafer top dielectric layer 415 is separately illustrated,covering the active area layer 410. Note that active area layer 410 may,and in most current applications probably does, include a number ofindividual dice, which for simplicity are not separately illustratedhere.

FIG. 7 b shows a second semiconductor wafer 420, which in thisembodiment includes a substrate 425 and an active area layer 430. Inthis case, the active area layer 430 includes a number of dice 431through 439. Note that, for convenience, only a limited number of diceare illustrated; the typical wafer may have many more (see, for example,FIG. 1). In preparation for dicing, saw kerfs 440 have been made betweeneach of the dice, through the active area layer 430 and extending intosubstrate 425. Also shown are a number of conductor-filled vias 445extending from the active area layer 430 downward into the substrate425. Each of these vias 445 is a slender, elongated recess that has beenfilled with a conductive material such as copper. Each is connected atits upper end with one or more components in the active area layer 430(although, strictly speaking, there is no requirement that every one ofthe vias 445 be so connected). Note also that these figures are notnecessarily drawn to scale.

A carrier 450 is then mounted to the top of second semiconductor wafer420. Carrier 450 may be any type of material on which the semiconductorwafer 420 maybe securely mounted and later removed. As should beapparent, carrier 450 helps to maintain the position stability of thedifferent parts of second semiconductor wafer 420 during the operationsthat follow. In this embodiment, those operations now include a backside427 reduction that, in effect, reduces the thickness of substrate 425. ACMP is then performed to expose the lower ends of the vias 445, as canbe seen in FIG. 7 c. The backside 427′ may at this point be cleaned. Anetching process is then performed in order to allow the lower ends ofthe vias 445 to protrude slightly from the further-reduced backside427″, as is shown in FIG. 7 d (note that some reference numbers havebeen omitted for clarity).

According to this embodiment, dicing now takes place to completelyseparate from each other the dice 431 through 439 of secondsemiconductor wafer 420. Stability and relative position of the dice maybe maintained during this procedure by first mounting them on a tape orother structure to which they will adhere. The individual chips known tobe good from previously-conducted testing are now selected, that is, thechips known not to be good are removed. A bonding tool 490 is attachedto the remaining chips, in this illustration chip 432, 434, 435, and438, as shown in FIG. 7 e. A chemical dip may then be performed, forexample using a citric acid or HCl bath, although the step is optional.

According to the embodiment of FIGS. 7 a through 7 h, the remainingchips 420′ of second semiconductor wafer 420 are then placed in a vacuumchamber 495, as shown in FIG. 7 f. A vacuum is drawn to create a vacuumenvironment, and then a vacuum-environment treatment is performed. Asnoted above, in accordance with the present invention, thisvacuum-environment treatment includes one or more of an H₂-based thermalanneal, an H₂-based plasma treatment, or an NH₃-based plasma treatment.The base semiconductor wafer 410 is then introduced into the vacuumchamber 495, and the remaining semiconductor chips 420′ may then bebonded to it, as shown in FIG. 7 g. As should be apparent, each chip432, 434, 435 and 439 is first aligned with an appropriate die or otherlocation on the surface of base semiconductor wafer 410.

The semiconductor device 400 is then removed from the vacuum chamber495, the bonding tool 490 is removed, and any necessary surfaceconditioning or cleaning may be performed at this time. Theconfiguration of semiconductor device 400 at this point in thefabrication process is shown in FIG. 7 h. The fabrication process maythen continue with the fabrication of additional components, theseparation of individual 3D-SIC devices, or other necessary operations.This may include, in some cases, the stacking and bonding of additionalchips on top of those components already formed, although this is notillustrated here.

Note that the sequence of operations described above in reference toFIGS. 4 through 7 h may be varied in any logically-permissible order. Inaddition, other operations may be added and, in some cases, existingoperations removed, without deviating from the spirit of the invention.That is, although the present invention and its advantages have beendescribed in detail, it should be understood that various changes,substitutions and alterations can be made herein without departing fromthe spirit and scope of the invention as defined by the appended claims.For example, the chemical dip prior to performing the selectedvacuum-environment treatment may be omitted entirely. For anotherexample, the good-die selection may be performed following thevacuum-environment treatment, or immediately after the chemical dip, ifperformed. And while it is preferred that the vacuum environment bemaintained for as long as possible as the operations described above areperformed, this is not a requirement of the invention unless explicitlystated, or apparent from the context.

In this way, among other advantages the chips of a 3D-SIC device may bemore reliably bonded together and provide for a longer Q-time withoutsignificantly increasing the costs of production. Note that noparticular result is required, however, unless explicitly recited for aparticular embodiment.

Finally, note that the scope of the present application is not intendedto be limited to the particular embodiments of the process, machine,manufacture, composition of matter, means, methods and steps describedin the specification. As one of ordinary skill in the art will readilyappreciate from the disclosure of the present invention, processes,machines, manufacture, compositions of matter, means, methods, or steps,presently existing or later to be developed, that perform substantiallythe same function or achieve substantially the same result as thecorresponding embodiments described herein may be utilized according tothe present invention. Accordingly, the appended claims are intended toinclude within their scope such processes, machines, manufacture,compositions of matter, means, methods, or steps.

1. A method of bonding chips for semiconductor devices, comprising:forming a plurality of chips on a first semiconductor wafer; forming asecond plurality of chips on a second semiconductor wafer, each chip ofthe second plurality of chips being compatible with a corresponding chipin the first semiconductor wafer; placing the second semiconductor waferin a vacuum chamber; drawing a vacuum in the vacuum chamber; performinga vacuum-environment treatment on the second semiconductor wafer; afterthe performing the vacuum-environment treatment on the secondsemiconductor wafer, aligning the second semiconductor wafer with thefirst semiconductor wafer; and heating the first semiconductor wafer andthe second semiconductor wafer to bond conductors disposed oncorresponding chips to each other.
 2. The method of claim 1, wherein theperforming the vacuum-environment treatment comprises using at least oneof a hydrogen (H₂)-based thermal anneal, a hydrogen (H₂)-based plasmatreatment, or an ammonia (NH₃)-based plasma treatment on the secondsemiconductor wafer.
 3. The method of claim 1, wherein the placing thesecond semiconductor wafer in the vacuum chamber is performed beforedrawing the vacuum.
 4. The method of claim 3, further comprising movingthe second semiconductor wafer from the vacuum chamber prior toperforming the vacuum-environment treatment.
 5. The method of claim 1,further comprising moving the second semiconductor wafer from the vacuumchamber into a bonding chamber prior to the heating.
 6. The method ofclaim 1, further comprising singulating the chips of the secondsemiconductor wafer prior to the heating.
 7. The method of claim 6,further comprising selecting only known good chips from the secondplurality of chips for bonding.
 8. The method of claim 7, wherein thechip selection is performed prior to performing the vacuum-environmenttreatment.
 9. The method of claim 6, further comprising mounting thesecond semiconductor wafer to a carrier.
 10. The method of claim 6,further comprising mounting the second semiconductor wafer to a tapeprior to singulation.
 11. The method of claim 1, further comprisingetching a bonding face of the second semiconductor wafer to raise aprofile of the conductors to be bonded.
 12. The method of claim 1,further comprising performing a chemical dip prior to thevacuum-environment treatment.
 13. The method of claim 12, wherein thechemical dip includes citric acid.
 14. The method of claim 12, whereinthe chemical dip includes hydrochloric acid (HCl).
 15. A method forfabricating a semiconductor device, comprising: forming a first chip ona first semiconductor wafer, the first chip having a target contact onits active surface; forming a second chip on a second semiconductorwafer, the second chip comprising a conductor-filled via extendingvertically downward from its active surface; mounting the secondsemiconductor wafer to a carrier; removing a portion of the waferopposite the carrier to expose the conductor-filled via; dicing thesecond semiconductor wafer; performing a vacuum-environment treatment onthe second semiconductor wafer; aligning the first chip and the secondchip; and bonding the exposed second-chip via to the first-chip targetcontact, wherein the bonding is performed after the performing thevacuum-environment treatment.
 16. The method of claim 15, furthercomprising performing a citric acid dip prior to the vacuum-environmenttreatment.
 17. The method of claim 15, further comprising performing anHCl dip prior to the vacuum-environment treatment.
 18. The method ofclaim 15, wherein the performing the vacuum-environment treatmentcomprises using at least one of a hydrogen (H₂)-based thermal anneal, ahydrogen (H₂)-based plasma treatment, or an ammonia (NH₃)-based plasmatreatment on the second semiconductor wafer.